(1) Field of the Invention
This invention relates generally to voltage converter circuits using charge pumps and relates more particularly to charge pumps, which have regulation and an active charge/discharge function.
(2) Description of the Prior Art
Charge pumps are circuits that generate a voltage larger than the supply voltage from which they operate. Charge pumps are a form of DC—DC converters that rely on capacitors instead of inductors for energy storage and transfer. The absence of inductors makes them attractive in situations requiring a low-power auxiliary supply. They use less circuit-board area, offer minimal component height, and are easy to use.
Charge pumps are switching capacitors circuits. The basic circuit is made from switches and capacitors. Charge pumps work as fixed multipliers, positive or negative, of the supply voltage. In order to generate twice the supply voltage two capacitors are required. One capacitor is a “flying” capacitor actually doing the pumping and one capacitor is acting as a reservoir, holding the generated voltage.
In order to generate three times the supply voltage two flying capacitors are required and one reservoir capacitor, in order to generate four times the supply voltage three flying capacitors are required and one reservoir capacitor, etc. for five times the supply voltage.
Charge pumps can have regulated or unregulated outputs. An unregulated charge pump either doubles or inverts the voltage that powers it and the output voltage is a function of the supply voltage. A regulated charge pump either boosts or inverts the supply voltage. Its output voltage is independent of the supply voltage.
Active discharge circuits in voltage charge pumps are normally used to shut down the circuit or reduce the output voltage to an initial value.
There are patents known using charge pumps within voltage converters:
U.S. Pat. No. 6,226,193 to Bayer et al.) describes a DC/DC converter operating on the charge pump principle, regulated to a fixed, predetermined output voltage and comprising two charge pump capacitors switched in a switch matrix consisting of nine switches. A control circuit is provided capable of controlling the switches so that the charge pump is changed over between a charging phase and a discharge phase and which is capable of operating the charge pump in two modes having different voltage gain factors (1.5; 2). The regulator comprises furthermore a regulator circuit capable of setting when the charge pump is active in the charging phase in the first mode the ON resistance of a switch present in the charge path of the charge pump capacitors or in the second mode the ON resistances of two switches present each in one of the charging paths of the first or second charge pump capacitor so that the charging voltage presented to the charge pump capacitors in the charging phase assumes the minimum value possible for a specific input voltage of the converter, a measuring circuit capable of defining the voltage occurring at one of the charge pump capacitors at a defined point in time of the charging phase; and a comparator capable of comparing the measured voltage to a predetermined voltage and then when the former attains the latter, changing over the charge pump into the corresponding other mode, wherein the predetermined voltage is selected so that the efficiency of the converter is optimized and the charging condition of the charge pump capacitors is not altered on changeover.
U.S. Pat. No. 5,841,648 to Mansfield) discloses an adjustable charge pump design employing a variable voltage level means to reduce the charge on the charge pump charging capacitor. The charge pump output is thereby controlled without the use of any secondary pass elements applied to the charge pump output voltage. The variable voltage level means, e.g. a variable resistor, “steals” voltage from the charging capacitor in either an inverting charge pump configuration or a doubling configuration. The voltage converter employing the adjustable charge pump is advantageously applied to providing an adjustable contrast control for an LCD display. Certain intrinsic safety requirements are achievable with the present design thereby making possible a backlit, intrinsically safe LCD display.
U.S. Pat. No. 5,680,300 to Szepesi to Park et al.) describes a charge pump controller providing the timing and drive voltages necessary for a self-regulated charge pump voltage converter. The controller utilizes a conventional non-overlapping two-phase clock to control the timing of switch drivers, which control an array of switches. Pump and reservoir capacitors are distributed within the array. One of the switch drivers produces a modulated, or linear, drive voltage for one of the switches in the array in response to the charge pump's output voltage. The controller thereby produces a regulated output voltage from the charge pump.